The occasional venous thromboses seen in patients with severe (homozygous) FXII deficiency are probably due to associated risk factors: a study of prevalence in 21 patients and review of the literature.

According to our personal experience and to the study of the literature, 11 cases of venous thrombosis have been described as sporadic reports in patients with severe (homozygous) factor XII (FXII) deficiencies. In every cases but 4, associated risk factors were found to be present (pregnancy, post-partum period, surgery, trauma, in dwelling catheter, AT deficiency, heterozygous factor V Leiden, Burger's disease). In some instances more then one condition was present. The four patients for whom no information is supplied, were cases gathered from old and logically incomplete files and therefore the existence of associated risk factors cannot be excluded. The papers which investigated the presence of venous thrombosis in cohorts of patients with homoxygous FXII deficiency demonstrated the occurrence of venous thrombosis in 2 additional cases out of a total of 63 patients investigated. In these latter cases thrombosis occurred during pregnancy. This brings the total number of patients with FXII deficiency who showed a venous thrombosis to 13. Only a few of these patients were investigated for the presence of concomitant congenital prothrombotic conditions. The conclusion of the study seem to suggest that the role played by FXII deficiency in the pathogenesis of venous thrombosis is minor, if any.